This invention relates to a transformer designed especially for underwater use, which transformer comprises a transformer core and its winding located in a first container filled with an insulating medium, the top surface of which container is provided with pressure-proof bushings for the connecting cables of the transformer, a second container filled with an insulating medium and means for equalising the pressure between the insulating medium in the second container and the transformer surroundings.
When pumping oil or gas, for instance, from the sea bottom, strict operating requirements need to be set on pumping equipment. The electricity supply of the pumps, for instance, is usually arranged by producing the electricity on a rig or a surface vessel from which it is transmitted to the pumps located as far as several kilometres away. To reduce voltage drops in the transmission cable, the voltage is usually raised to a medium voltage and only transformed close to the consumption point to the operating voltage of the motors running the pumps, typically to a level of 1 kV. The structure of such a pump must be such that it is capable of functioning in and enduring conditions at at least 500 m below the surface of the sea.
Prior art uses an oil-filled transformer whose container is made of special steel. Such an underwater transformer is equipped with a pressure equaliser which may slightly leak due to diffusion or malfunction. In such a case, the insulating medium fluid, typically oil, leaks into the sea already causing environmental hazards as such, but the water which has at the same time leaked into the transformer container also weakens the electrical insulation of the transformer and damages the transformer on the long run, in which case electricity supply is interrupted and a sudden pressure increase caused by an electric arc can push all the oil in the transformer into the sea.
British Patent Publication 1 604 978 discloses a solution in which a second oil container with a connection to the pressure equaliser is located below the transformer container. Between the containers, there is a bellows which allows the transformer oil to thermally expand in the first container. This solution provides the advantage that the same pressure exists on both sides of the transformer container, in which case its structure can be made light. In addition, the bellows structure prevents water from leaking into the first container in which it may damage the transformer insulation. In the solution in question, the electrical connection is led directly into the inner container in which the transformer core and its winding is suspended.
Japanese Patent Publication 57 018 306 discloses a double-walled transformer container. A bellows is also used to equalise the pressure between the inner transformer container and the space between the walls, and also to prevent the oil from getting into contact with water.
A particular disadvantage of both above-mentioned solutions is that the electrical inlets must be led directly through to the inner container, whereby their leaks easily become a critical.
It is an object of the present invention to introduce a transformer which is better suited than the known transformers to be located at the bottom of the sea. A further object is to produce a transformer construction in which conventional standard-structure distribution transformers can be used as far as possible and to thus achieve low manufacturing costs.
The above-mentioned objects are achieved by means of a transformer of the invention, characterized in that in it a second container is arranged to fully encompass a first container and that the second container is equipped with pressure-proof cable bushings and associated connectors for external electrical connections of the transformer. In the transformer of the invention, the core and windings of the transformer are thus located in the inner container which, in practice, can be a completely standard-structure transformer which is, however, completely encompassed by an outer container also filled with an insulating medium. The pressure-proof bushings preferably arranged in the first container comprise bushing insulators, and the connecting cables of the transformer are connected to these bushing insulators with pressure-proof and water-proof cable shoes. The transformer can be made very reliable by applying this procedure.
According to the invention, the wall of the first container comprises corrugated parts to allow for the volume changes caused by the thermal expansion and pressure changes of the insulating medium filling the first container. Further, it is advantageous that means for equalising the pressure between the insulating medium in the second container and the transformer surroundings comprise a pressure equalising container arranged on top of the second container and a pressure equalising pipe connected thereto, which pipe is led through the second container through a pressure-sealed inlet on its upper surface and arranged to extend to the bottom part of the second container prior to opening into the second container.
It can be noted that the transformer of the invention provides the advantage that the transformer itself can be of standard structure, in which case the wall structure of the first container, i.e. the wall structure of said standard-structure transformer, is corrugated allowing the oil to thermally expand, in which case no separate bellows is needed for this. Further, in the transformer of the invention, a leak in the pressure equaliser does not cause the filling up of the entire outer container with water, since the end of the pipe is led close to the bottom of the outer container. In such a case, a minor leak in the pressure equaliser results in that water goes directly to the bottom part of the outer container and thus does not affect the cable shoes. A leakage water of this kind can only cause a risk with the bushing insulators when the outer container is nearly full of water. Even after this, only a damage in the first, i.e. inner, container or a leak in the watertight cable shoe results in water entry inside the first container and thus damages the insulation of the transformer and causes a disruptive discharge.